The tropics are undergoing profound, rapid landscape change. Humid tropical regions occupy about a quarter of Earth’s land surface, yet they contribute a substantially higher fraction of the water, solutes, and sediment discharged to the world’s oceans. Nearly half of Earth’s population lives in the tropics where development stresses can potentially harm soil resources, water quality, water supply, and increase landslide and flood hazards.
“As our influence on Earth’s lands and waters grows more substantial, we need to understand and quantify these effects to preserve our well-being,” said Matt Larsen, USGS Associate Director for Climate and Land Use Change. “Earth sciences are central to meeting this need because they guide the monitoring, assessment, and modeling of natural processes that help us to understand our interaction with the environment.”
Puerto Rico is representative of rapid, ongoing change in the tropics. Many agricultural lands are undergoing reforestation, while coastal areas are becoming heavily urbanized. The area is also changing because of the introduction of nonnative species, water supply projects, and the construction of roads and other kinds of infrastructure. Slower, larger scale factors are also at work in environmental change, such as the deposition of airborne pollutants and natural and human-induced climate change.
In eastern Puerto Rico, four humid-tropical watersheds exhibit varying types of underlying geology while their waters flow through or near the mature forest of the Luquillo Experimental Forest (also known as the El Yunque National Forest). Lying within a relatively small geographic footprint, the four watersheds provide a natural laboratory that allows scientists to quantify and characterize landscape change in this region.
In recognition of this opportunity, scientists from the USGS have conducted research in eastern Puerto Rico for several decades, in collaboration with scientists from the U.S. Forest Service. The scientists’ aim is to study hydrologic, geologic, geomorphic (Earth-shaping), and anthropogenic (human-driven) processes in the area with the goal of generalizing their investigation to the broader tropical world.
These findings have recently been published by the USGS. Because eastern Puerto Rico resembles many tropical regions in terms of geology and patterns of development, implications from this study are transferable to other tropical regions facing deforestation, rapid land-use change, and climate change.
Location, location — and precipitation
Focusing on four small watersheds in eastern Puerto Rico allowed the scientists to integrate studies of hydrologic and chemical processes. Two watersheds are located on coarse-grained granitic rocks; two are located on fine-grained volcanic rocks. For each bedrock type, one watershed is covered with mature rainforest. The other two watersheds, like most of the rest of Puerto Rico, were subjected to intensive agriculture in the 19th and early 20thcentury, but they have been undergoing reforestation as a result of a shift from an agricultural economy to an industrial one and the related human migration to urban areas.
Owing to the island’s steep topography, low water-storage capacity, and dependence on trade-wind precipitation, Puerto Rico’s people, ecosystems, and water supply are vulnerable to extreme weather such as hurricanes, floods, and droughts. At the same time, rainfall varies greatly over small distances in eastern Puerto Rico, based on differences in elevation, topographic position and aspect, and proximity to the ocean.
Puerto Rico lies directly in the path of the easterly trade winds, which deliver steady rainfall to the mountains and steer weather systems called tropical waves toward the island. Hurricanes and tropical storms derived from these systems typically deliver the majority of yearly rainfall, but northern cold fronts can also deliver heavy rainfall for several days at a time, usually from December through February.
These rain events vary greatly in frequency and intensity, contributing to substantial differences in annual precipitation. A unique aspect of this study is that USGS scientists sampled and chemically analyzed river water during major storms using automated equipment. Such intense storms had never before been studied in this manner.
The largest storms can have profound geomorphic consequences, such as landslides, debris flows, and deep gullying on deforested lands. Past deforestation and agricultural activities in the developed watersheds led to greatly accelerated erosion and soil loss. Rates of erosion in two forested watersheds were considerably higher than expected, presenting a new area for future study.
Regional weather patterns and the sources of air masses influence the type and timing of atmospheric contributions to the land. Eastern Puerto Rico receives marine salts and Saharan Desert dust in rainfall, while nitrogen loads in precipitation have roughly doubled since measurements began in 1985.
A matrix of factors in landscape change
The 15-year Water, Energy, and Biogeochemical Budget dataset, including data on stream discharge, field parameters, suspended sediment, and nutrients, is available from the USGS National Water Information System (NWIS). The dataset provides a baseline for characterizing future environmental change in eastern Puerto Rico and will improve our understanding of the interdependencies of land, water, and biological resources and their responses to changes in climate and land use.
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